The present invention relates in general to pneumatic separators for separating heavier material from lighter material, such as, for example, the stems from fragments of tobacco leaves, and more particularly to a recirculating pneumatic separator for tobacco stemmery operations for separating tobacco leaf fragments from stems of tobacco leaves.
Functions normally performed in a tobacco stemmery operation where whole leaf tobacco is processed after leaving the auction warehouse floor are ordering and conditioning of the tobacco leaves, threshing or removal of the lamina from the stem, separation of the lamina and stems, drying, and packing. The present invention is concerned with machinery for ordering and conditioning the tobacco leaves, threshing or removal of the lamina from the stems, and separation of the lamina and stems. The drying and packing of the tobacco leaf fragments is then accomplished by other machinery.
Ordering and conditioning is normally accomplished by tumbling the tobacco in a rotating cylinder with a slightly declining axis of rotation while spraying the tobacco with steam and water. The object is moisten the tobacco making it more pliable and to raise its temperature for the same reason.
Threshing is accomplished by a rotating cylinder with pieces of flat steel (teeth) projecting from the cylinder surface. The axis of rotation is horizontal with the tobacco evenly fed across the upper surface of the cylinder. The teeth grab the leaves, cutting some and tending to tear the lamina from the stems. At the same time the tobacco is thrown radially outward by centrifugal force where it contacts a close fitting concave or basket which encloses the lower portion of the rotating cylinder. The concave has openings through which the tobacco may pass. In the early stages of threshing these openings are fairly large. A standard basket may be a sheet of 1/4 inch plate rolled to just clear the teeth. This rolled shape is then cut by torch into a series of diamond shapes with a 21/2 inch slot between each row of diamonds. The slots run in the direction of rotation and the diamonds overlap. As the tobacco passes through there is additional cutting of stems and removal of lamina.
Threshing is not 100 percent efficient so several threshers are used with a separator or separators in between. Latter threshers have smaller openings in the concaves and the last one may have 11/2 inch to 2 inch round holes.
After separation the lamina and stems are separately dried down to 10-11 percent moisture, cooled and packed into containers by large hydraulic presses for storage for a year or more. During storage, some of the natural starches convert to sugars by natural fermentation.
The machine of the present invention is similar to the prior art recirculating pneumatic separator described in Rowell U.S. Pat. No. 3,608,716, which is designed to receive tobacco from the thresher in an even flow into a regulated uprising air column such that the lighter free lamina rises upward with the air stream and the heavier stem or stem with lamina attached, frequently referred to as flags, fall downward against the air flow to be ejected to the next thresher or another separator. In the machinery of the type disclosed in the prior U.S. Pat. No. 3,608,716, a mixture of free lamina, attached lamina and stem, and clean stem is pneumatically conveyed from the thresher or previous separator into the screening separator. Here the air passes through a rotating screen of perforated metal and is returned to the fan. The tobacco mixture falls down the back wall into a four bladed rotating air lock which passes the tobacco out of the air stream.
Immediately it falls into a high speed six bladed rotating inlet rotor. These blades with resilient tips impart a uniform velocity to the tobacco and hurl it into a chamber, shown at 7 in the patent. A reasonably even air flow uprising through the chamber is induced by a fan or fans.
The lighter lamina or strip, as it is commonly called, has lesser inertia and is effected by the air stream immediately and starts on its journey upward. Stems and flag are less effected by the air since they are heavier and continue more or less to the wall opposite the inlet. An adjustable damper called a ski-jump is located at the lower end of this wall, blocking air flow next to the wall to create a dead air space.
Without air flow, the stem and flag fall to the ski-jump and due to its angularity are directed in the opposite direction and slightly downward across the air stream for the second pass. Some free lamina may be jarred loose and be re-entrained in the air stream.
The lighter material enters the strip discharge section and is locked out to an inspection belt. Air exits each end of the screen, passes through dampers and enters the suction of one or the other of two internal fans.
The internal fans blow into the bottom of the separating section, through a louvered screen to even the air flow and then upward through a perforated belt. The belt serves as an additional air flow equalizer and prevents the heavy material from falling into the bottom of the separator.
The reason for a belt is to provide a means of keeping the perforated holes from stopping up due to small material that may pass through the internal screen seals and be held on the underside of the perforated surface. Using a perforated belt insures that the under surface is always changing to become an upper surface allowing any material to be blown free.
Having completed the second pass across the air stream the heavy material slides across the perforated belt into a pneumatic pick-up to be conveyed to the next thresher or the next separator.
In order to keep the system negative in regards to air pressure, air is bled from the discharge of the fans which convey between the machines (transport fans). All leakage is now on an inward direction which helps to contain the dust and dirt. The make up air enters the machine through holes along each side at the belt level helping to keep the belt seals clean plus air is admitted at the upper end of the belt and blows along the belt surface to assist the heavy material out of the machine.
Air is also exhausted from between the belt from a trough located under the upper surface of the belt. The trough creates a dead air space which allows sand or other material to fall from the belt surface.
Too high air velocity in the separating chamber will cause stems and flags to go with the good strip. Too much stem is not desired and large pieces (called objectionable) are just that. Too low a separating velocity lowers the stem content of the separated strip but allows too much good threshed material to pass on to the next thresher or separator. This results in further degradation of the strip and is to be avoided.
Separating air flow is grossly regulated by slide or louver dampers at the suction side of the fans and is finely tuned by use of a single bladed by-pass damper.
Separated strip after leaving the air stream passes through the discharge lock and falls to a belt for delivery to an inspection and collecting belt.
While the philosophy in the tobacco threshing and separating community until a few years ago was to thresh increasing quantities of tobacco and produce a more uniform product (:.e. strip) with less fines and, because of the higher loads, less larger particles, the present approach is toward more larger pieces which means once a particle is threshed from the stem, it should be separated at once without being subjected to additional threshing or handling resulting in degradation of size.
While the recirculating pneumatic separator of the above identified patent met the former requirements, the air jet impeller and air diffusion grating features do not produce the results now desired. It is now recognized that use of a high velocity air stream, while separating more of the good material, has the drawback of also carrying along stem, flag and unthreshed leaves leading to an unacceptable high stem content. However, if a high velocity air stream is used to separate, and then the air is slowed to drop stem, a high separating efficiency and low stem content can be achieved. By use of a novel air jet nozzle arrangement, the apparatus of the present invention is now able to direct the material coming off the air jet impeller vane into a high arc trajectory, so that the jet acts to break tangles or dislodge particles, and the high arc trajectory allows more time for the separating air to interact so that, as the heavier material takes a more downward course at the end of its trajectory, it falls into the area of highest velocity. Because the tobacco is moving downward against the upward air flow, the relative air velocity is increased without an increase in air flow further enhancing the separation. Also, it has been recognized that tobacco, as it enters the threshing separating portion of its processing, contains a very high proportion of sand or silica stuck to or embedded in the tobacco. Threshing and separating tend to dislodge this very abrasive medium and it is desirable to remove it. The apparatus of the present invention includes, as part of the air circuit, a large plenum or settling chamber designed to allow this sand to settle out of the air stream prior to entering the fans. This makes it possible to use lighter construction fan wheels of higher efficiency with considerable savings to the user in power cost.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.